1. Field of the Invention
The present invention relates to a vertical (i.e., vertical-electrode type) GaN-based light emitting diode (LED) and a method of manufacturing the same. In the vertical GaN-based LED, when a negative electrode (n-electrode) contacts an n-type GaN layer from which a sapphire substrate has been removed by a laser lift-off (LLO) process, a contact resistance and an operating voltage are reduced to enhance a current diffusion effect.
2. Description of the Related Art
Generally, a GaN-based LED is grown on a sapphire substrate, but the sapphire substrate is a rigid nonconductor and has poor thermal conductivity. Therefore, there is a limit in reducing the manufacturing costs by reducing the size of a GaN-based LED, or improving chip characteristics such as optical power and electrostatic discharge (EDD). Particularly, because application of a high current is essential for achieving high power LED, it is important to solve a heat-sink problem of the LED. To solve this problem, there has been proposed a vertical GaN-based LED in which a sapphire substrate is removed using a laser lift-off (LLO) process. Hereinafter, a vertical GaN-based LED according to the related art will be described with reference to FIGS. 1 and 2.
FIGS. 1 and 2 are sectional views illustrating a method of manufacturing a vertical GaN-based LED according to the related art.
Referring to FIG. 1, a buffer layer 110, an n-type GaN layer 120, an active layer 130, and a p-type GaN layer 140 are sequentially grown on a sapphire substrate 100. A positive electrode (p-electrode) and/or a reflective layer 150 and a conductive adhesive layer 160 are sequentially formed on the p-type GaN layer 140.
Thereafter, a predetermined temperature and a predetermined pressure are applied to the conductive adhesive layer 160, thereby attaching a silicon substrate 170 onto the conductive adhesive layer 160. The silicon substrate 170 may be replaced by a copper tungsten (CuW) substrate or a metal substrate. The metal substrate can also be referred to as “metal structure support layer”.
Referring to FIG. 2, an LLP process is performed to sequentially remove the sapphire substrate 100 and the buffer layer 110, thereby exposing the top surface of the n-type GaN layer 120.
Thereafter, an n-electrode 180 is formed on the exposed surface of the n-type GaN layer 120, and laser scribing, wet etching, or dry etching is used to perform a device isolation process. Alternatively, the n-electrode 180 may be formed after the device isolation layer.
However, in the vertical GaN-based LED according to the LLO process, the surface of the sapphire substrate 100 is pre-treated using ammonia (NH3) gas before the buffer layer 110 is grown on the sapphire substrate 100. Therefore, the exposed surface of the n-type GaN layer 120 is formed to have the structure of a GaN polarity with the [000-1] direction of a wurtzite structure, that is, the structure of an N-face polarity in which gallium elements are disposed on a vertical uppermost layer of nitride (N) elements (see FIG. 3(a)).
When the n-electrode 180 containing aluminum (Al) is formed on the surface of the n-type GaN layer 120 with the N-face polarity, an AlN layer serving as a piezoelectric layer is formed at an interface between the n-type GaN layer 120 and the n-electrode 180.
However, when the AlN layer is formed at the interface between the n-type GaN layer 120 and the n-electrode 180, a piezoelectric effect is directed toward the n-electrode, thereby increasing the contact resistance. This is well known to those skilled in the art by many conventional documents (see APPLIED PHYSICS LETTERS Vol. 79 (2001), pp 3254-3256, “Crystal-polarity dependence of Ti/Al contacts to freestanding n-GaN substrate” and APPLIED PHYSICS LETTERS Vol. 80 (2002), pp 3955-3957, “Characterization of band bendings on Ga-face and N-face GaN films grown by metalorganic chemical-vapor deposition”).
Also, the n-type GaN layer 120 is formed by implantation of n-type impurities (e.g., Si) into an undoped GaN layer and thus has a high doping concentration.
However, when the n-type GaN layer 120 has a high doping concentration, current crowding occurs only at a lower portion of the n-electrode 180 contacting the n-type GaN layer 120 and a current does not diffuse over the entire active layer 130. Consequently, the light-generation efficiency of the LED is degraded and the lifespan of the LED is reduced.